An optical fiber is typically used to transmit coherent monochromatic light, which is emitted from an output end of the optical fiber, hereinafter called the emitting tip. Such optical fibers typically have an active core diameter of about 6-20 microns, in the case of a single-mode fiber. A fiber optic collimator is a common optical node found in many places in the modern fiber optic industry. The collimator is a device which holds in an adjustable manner the emitting tip of the optical fiber so that it is positionable in multiple axes near the focus of a lens, so as to provide at the output of the lens (collimator) a parallel (collimated) laser beam. Such positionability is typically expressed as an adjustability with multiple degrees of freedom, such as plus and minus translation (Δ) in X, Y and Z linear axes, Rotation (Ω) about the longitudinal axis of the collimator and Tilt of the emitting tip of the optical fiber (hereinafter called Tip-Tilt), leading to a requirement of having multiple degrees of freedom (preferably nine), each degree of freedom decoupled from the other, for establishing a precise alignment of multiple collimators in an array.
FIG. 1 illustrates a schematic view of a typical fiber optic collimator. A fiber-optic cable 600 has an emitting tip 601 that is to be positioned at a lens focus point 000. The light beam exiting emitting tip 601 is a divergent beam 001, that is, the light beam diverges the farther away it travels from the emitting tip 601. A lens 111 is positioned at a distance F (focal length) from the emitting tip 601 so as to provide at the output of the lens a collimated light beam 002.
The position of the emitting tip 601 relative to the lens focus point 000 strongly influences the beam parameters. For example, a negative displacement of the tip along the longitudinal axis of the collimator leads to convergence (focusing) of the beam as shown by the arrows adjacent beam 002 in FIG. 2A, while a positive displacement of the tip along the longitudinal axis of the collimator leads to divergence (de-focusing) of the beam, as shown by the arrows adjacent beam 002 in FIG. 2B.
A tilt of the fiber-optic cable 600, as noted above called a Tip-Tilt (or an equivalent effect caused by deviation of beam propagation relative to an angled cleaving of the emitting tip 601), leads to a perpendicular displacement delta (Δ) of the beam centroid from the optical center of lens 111, as shown by the arrow adjacent beam 002 in FIG. 4. The effect of such displacement is that the maximum intensity of Gaussian-shaped divergent beam 001 no longer coincides with the center of lens 111, thereby decreasing the fill factor and quality of the collimated laser beam 002.
A lateral shift of the emitting tip 601 relative to the focal plane of lens 111 by an amount delta (Δ) Y as shown in FIG. 5, induces an angular deviation of the output beam 002, which can easily result in missing a target or receiver area that is located at a remote distance from lens 111.
FIG. 5 illustrates another parameter that requires adjustment, namely the polarization angle Omega (Ω) of each collimated beam 002, which should be adjustable so as to match the polarization angle Omega of each other beam in an array of collimators. Thus, the fiber optic 600 should be angularly adjustable about the Z axis of the collimator so as to adjust beam polarization.
From the above, it should be clear that the ability of precisely control adjustment of the position of the emitting tip in the collimator is an essential requirement for a collimator, For example, at an aperture lens diameter of 30 mm and a focal length of 150 mm, a diffraction limited target (˜3 cm) at a distance of 1000 m will be totally missed if the emitting tip 601 in the collimator is displaced by only 5 microns from the focus 000 of the lens.
Numerous mechanical controlling stages (like X-Y-Z or tip-tilt-rotation nodes) have been developed in the fiber optic industry to provide precision control of the position and orientation of a fiber node that supports therein the fiber emitting tip. Such mechanical devices are typically supplied with micrometers as well as with stepping motors and servo motors so as to allow precise manual or computerized alignment of the node that holds the fiber optic emitting tip and other parameters of the collimator, for alignment in the X, Y and Z directions of the tip, as well as its rotational position, relative to the lens focal point in the controlling stage. Computerized alignment is particularly important in view of the fact that existing fiber optic controlling stages typically require iterative adjustment, since adjustment along one axis typically affects to some degree alignment in another axes, that is such adjustments heretofore have not been decoupled one from the other.
Thus, existing alignment controlling stages are bulky and require a lot of space to provide the necessary stiffness and accuracy to enable such precise control, and, as rioted above, have adjustments that are somewhat “coupled” one to the other in that adjustment of one alignment stage disadvantageously affects the alignment of another stage. Thus, in combination with the controlling stages, the physical space taken up by a collimator extends substantially beyond the perimeter required for transmitting just the light beam. Advanced optical systems, such as systems requiring more power, however, may require more than use of a single fiber optic collimator, and therefore an array of collimators mounted so as to be in close proximity and parallel to each other is also highly desirable. For example, FIG. 6 illustrates coherent laser beam combining in a sparse aperture array design that requires dense packing within a mount of a plurality of collimators, such as the seven fiber optic collimators shown. Arrays having even more fiber optic collimators would be desirable.
However, the density of packing is among the most difficult requirements to meet for achieving the highest optical performance in a fiber optic collimator array, The array may have standard fiber optic ferrules with static positioning of the fiber outputs, or may have fast responding fiber positioners with high frequency bandwidth, providing computer controlled compensation of deviations of separate beams from the target induced with vibrations or/and optical turbulences in a propagation media (e.g., atmosphere).
The physical closeness of the collimators to each other in a densely packed array of collimators makes alignment of the separate fiber outputs in a common mount a complicated task. This is due to the spatial requirement of the need for sufficient access to a means used for adjustment of the alignment provided by each of the collimators. Moreover, after an optimum alignment of each collimator in the array is found, the alignment should hold, that is, be very stable under changing environmental conditions, such as vibration, temperature, etc.
One known design for a fiber collimator comprising alignment controlling stages is shown by my prior publication “Development of Adaptive Fiber Collimators for Conformal Fiber-Based Beam Projection Systems”, published in the Proceedings of SPIE, Vol. 7090, 709008 (2008), incorporated herein by reference. The mechanical design of the collimator comprises four cylindrical shaped, concentric elements or alignment nodes. Although that design functioned according to specifications to provide the necessary controls of alignment, alignment was somewhat cumbersome because adjustment of alignment in one or more of the axes via one node was not decoupled from affecting alignment in one or more of any of the other axes in another node.
Alignments needed are:                Course adjustment of tip 601 along the optical axis Z for alignment with respect to the point 000 in the collimator. FIGS. 2A and 2B illustrate the focusing and defocusing alignment effect, respectively of the output beam, as a result of tip displacement along the optical axis.        Course adjustment of tip 601 along the optical X and Y axes for alignment with respect to the point 000 in the collimator. FIG. 4 illustrates the effect on beam alignment resulting from a lateral shift (X-Y) of the output beam.        Rotation of the fiber 600 around its longitudinal axis. FIG. 5 illustrates the effect of rotation of fiber optic 600 by an angle Omega (Ω) and a corresponding rotation of its polarization plane 010, so that should the collimator be part of collimator array, the light beam of each collimator of the array can be adjusted to have the same polarization.        Corrections for deviations (tip-tilt) of the fiber 600 relative to the point 000. FIG. 3 illustrates the effect of tilt-tip deviation of the optical fiber 600, thereby establishing a need for x and y axis alignment of the tip 601 with respect to the point 000 during a tip-tilt condition.        And finally, precise control of the displacement of the tip 601 in the X and Y axis with respect to the focal plane of lens 111. FIG. 4 illustrates the effect of displacement in the Y axis direction.        
Therefore, there is a need in the art for a method and apparatus for providing a fiber optics collimator having multiple, up to nine, degrees of freedom to adjust its alignment in a decoupled manner and which is suitable for mounting in a densely packed array. Suitability for mounting in a densely packed array is judged by a reduction in the spatial requirements of the means for adjusting the collimator, thereby allowing dense packing and access to the alignment adjustments.